In general, in a printed circuit board, a decoupling capacitor is amounted in order to secure a sufficient power integrity. The decoupling capacitor is used in order to suppress a variation in power supply voltage which is supplied to an LSI on the printed circuit board. The decoupling capacitor is connected between a power plane and a ground plane in the printed circuit board. The impedance (power supply-ground impedance) between the power plane and the ground plane is reduced by the decoupling capacitor. Accordingly, the decoupling capacitor is capable of functioning to improve the power integrity or, in other words, functioning to stabilize the power supply voltage which is supplied to the LSI. A power supply voltage variation ΔV on the printed circuit board can generally be expressed by the following equation:ΔV=Z×I 
where Z is the power supply-ground impedance, and I is the electric current which is drawn from the power plane by the LSI. With the increase in speed of the LSI, there is a tendency that the current I becomes larger. Thus, recently, the measure for reducing the power supply voltage variation ΔV has become very important.
The power supply voltage variation ΔV needs to be suppressed to a range less than the maximum tolerable voltage drop (also referred to as “maximum tolerable noise voltage”) which is stipulated by the specifications of the LSI. For this purpose, it is necessary to lower the power supply-ground impedance Z to below a target impedance ZT by disposing the decoupling capacitor on the printed circuit board.
The target impedance ZT can generally be expressed by the following equation:ZT=maximum tolerable voltage drop/I 
where the maximum tolerable voltage drop is expressed by a product between a power supply voltage V and a tolerable voltage error %.
Jpn. Pat. Appln. KOKAI Publication No. 2006-352059 discloses a digital signal processing board on which a decoupling capacitor, which is connected between a power supply input line and a ground, is mounted. In this digital signal processing board, as the decoupling capacitor, use is made of an electrolytic capacitor having an equivalent series resistance of 25 mΩ and an equivalent series inductance of 800 pH.
As has been described above, the decoupling capacitor is useful in improving the power integrity of the printed circuit board. However, strictly speaking, the power supply-ground impedance of the printed circuit board is different between when a high-side transistor in a switching power supply, which is mounted on the printed circuit board, is turned on, and when a low-side transistor in the switching power supply is turned on. The power supply-ground impedance at the time when the high-side transistor is turned on is higher than the power supply-ground impedance at the time when the low-side transistor is turned on. The reason for this is that the coil in the switching power supply functions as an inductor when the high-side transistor is turned on, whereas the coil functions not as an inductor but as a power source (resistor) when the low-side transistor is turned on. The impedance of the inductor increases as the frequency becomes lower. Thus, in a low frequency region, the power supply-ground impedance at the time when the high-side transistor is turned on becomes higher than the power supply-ground impedance at the time when the low-side transistor is turned on.
In this manner, the power supply-ground impedance varies depending on the operation state of the switching power supply which is mounted on the printed circuit board. The operation state of the switching power supply, that is, the ratio of the ON time of the high-side transistor to the switching cycle, varies depending on the amount of current consumption of a device, such as a processor, which is mounted on the printed circuit board. In a printed circuit board on which a device, such as a high-capability processor, is mounted, the ratio of the ON time of the high-side transistor is relatively high. On the other hand, in a printed circuit board on which a device, such as a low-capability or middle-capability processor, is mounted, the ratio of the ON time of the high-side transistor is relatively low and, in some cases, the ON time of the low-side transistor occupies the major part of the switching cycle.
However, in the conventional design of the printed circuit board, no sufficient consideration has been given to the operation state of the switching power supply. It is thus possible that an excessive number of decoupling capacitors, or a decoupling capacitor with an excessive capacitance, is disposed in order that the power supply-ground impedance may be set to be lower than a target impedance over the entire frequency band.